1. Field of the Invention:
The present invention relates generally to a solid state overcurrent detector for detecting overcurrent, for instance, of a power line or load, thereby to monitor same. It is further applicable to protection of AC power lines, and a circuit interruptor using the same, and more particularly concerns an overcurrent detector and circuit interrupter suitable for most appropriate protection of AC power lines.
2. Background of the Invention:
I have proposed a solid state overcurrent detector comprising a microcomputer therein. In the proposed solid state overcurrent detector, a time-current characteristic for detection of overcurrent for the power line or the load is based on data stored in a ROM in the microcomputer. The above-mentioned time-current characteristic is generally of inverse characteristic as shown in FIG. 1 in order to protect the power line or the load to assure as stable an electric power service as possible. The general characteristic shown in FIG. 1 is determined by considering heat resistance capacity of the power line and the load, and interrupting characteristic of fuses in the upstream side.
Hitherto, the apparatus for producing such a characteristic is configurated by comprising current sensor means for sensing the overcurrent, sampling means for sampling the output signal of the current sensor means with a predetermined sampling rate, level discrimination means for discriminating level of the output signal of the current sensor means and time-current operation means for carrying out time-current operation based on the inverse time-current characteristic as shown in FIG. 1, wherein in a current pickup characteristic line F a part A shows a long-time characteristic, a part B shows a short-time characteristic, a part C shows an instantaneous characteristic, and a curve D shows an interruption characteristic of a fuse in an upstream side of the power line and a curve E shows a heat resistance characteristic of the power line or a load. In such an apparatus, we have proposed to store data for the characteristic for responding a restoration to a normal state of operation within a predetermined time for the case that the overcurrent changes in the program of the apparatus taking account of heat discharging characteristic of the power line and load.
In general, it is known that heat discharging characteristics of the electric power lines and loads are known as have a temperature attenuating by an exponential function or the like characteristic. Accordingly, by carrying out processing in the microcomputer of the exponential attenuation characteristic or by storing results of a known operation preliminarily in a ROM as data table, an apparatus having a capability of responding to overcurrent change can be realized.
The above-mentioned way of carrying out data processing taking account of the heat discharging characteristic under a program utilizing the data table has the following problem. Firstly, in executing operation under the program, a very complicated program, requiring an enormous memory space, is necessary for obtaining a very accurate heat discharging charging characteristic. Therefore, the processing time becomes very long, and a most appropriate protection of the power line and load by the apparatus becomes difficult. In another apparatus, having the data table in the ROM to obtain the result of the above-mentioned operation, though the program as such can be made relatively short, the amount of data becomes enormous for obtaining accurate heat discharging characteristic, and therefore the ROM must have a large capacity. In case a one-chip microcomputer is used as the apparatus, obtaining the most appropriate characteristic becomes very difficult since the capacity of the ROM is limited and no sufficient data can be stored therein.
Another important problem of the microcomputer-implemented overcurrent detector is that, when the power supply is temporarily interrupted or a surge noise or the like disturbance comes, the microcomputer is liable to be involuntarily reset. After such resetting, the whole process must be reinitiated, and data produced at the time of the undesirable stopping is lost as a result of the stop of the this power supply termination. This is a grave problem regarding a fundamental function of the overcurrent detector or circuit interrupter using the overcurrent detector. Furthermore, when a secondary output current of a current sensor means, such as a current transformer, is used as a power source for the microcomputer, the above-mentioned losing of data or stopping of the operation of the circuit is an important problem. This is because when no overcurrent or normal current flows in the power lines, the power source for the microcompouter stops its feeding, and accordingly, the abovementioned re-starting of the processing circuit starts each time when the overcurrent or the normal current flows in the power line. Accordingly, protection against accumulated heat, that is, issuing of a detection signal responding to heat accumulation due to overcurrent, can not be carried out at all.